An object detecting apparatus is known that it projects a pulse of a laser beam outward and then detects the laser beam returned back which is reflected by an object thereby detects the distance and the object which is located on the optical path of the laser beam. Such object detecting apparatus is called LiDAR (Light Detection and Ranging).
Recently, LiDAR is beginning to be used in a field so-called autonomous driving. It often compensates such disadvantages, for examples, a camera sensor is susceptible to external illumination environment and a milliwave radar is with a low resolution. Such LiDAR is sometimes combined with a camera sensor or a milliwave radar to accurately detect a relatively small obstacle under driving environment.
One example of LiDAR structure used in the autonomous driving is disclosed in the patent literature 1 (PTL1). In the LiDAR disclosed in PTL1, a near-infrared laser as the light source and a light detecting element as the receiver establish a pair which is located on an electrical circuit board to match the measuring direction. To obtain the high resolution distance information in the field of view, it uses thirty two or sixty four pairs. Therefore, the cost of the apparatus is very high.
Another example of LiDAR is disclosed in non-patent literature 1 (NPL1). The LiDAR disclosed in NPL1 has a rotating polygon mirror with three surfaces where each surface has a different inclining angle. The polygon mirror deflects a laser beam and then projects the laser beam within a total vertical angle with 4.5 degree. The reflected light from the object is returned to the same surface of the polygon mirror again and collected to the light-detecting element for the object detection.
The advantage of the LiDAR disclosed in NPL1 is that it can detect the reflected light from different vertical positions only by single light detecting element. However, since the reflective surface of the polygon mirror has the different inclined angles, the design of the gravity center of the rotating polygon mirror becomes quite difficult and it brings about an extra problem of the increased cost. When the polygon mirror is running in high speed rotation for a long time, the heat generation and the friction of the bearing bring about the maintenance problem in the circumstance of the long-time usage.
The LiDAR using the rotating mirror is disclosed also in non-patent literature 2 (NPL2), but there is no detailed description of the configuration in NPL2.